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May 5, 2011 Office of Environmental Information Docket (OEI) Docket ID: EPA-HQ-ORD-2011-0050 Mailcode: 2822T U.S. Environmental Protection Agency 1200 Pennsylvania Avenue NW Washington, DC20460 

## Karen StJohn Senior Director Regulatory Affairs 

# MAY 0g 2011

SUBJECT: BPAmerica Comments on the "Integrated Science Assessment of Ozone and Related Photochemical Oxidants', EPA Document EPA/600/R-10/076A, Released March 2011 BP America (BP) appreciates the opportunity to provide written comments to the U.S. Environmental Protection Agency (EPA) on the Integrated Science Assessment (ISA) of Ozone and Related Photochemical Oxidants. 

### BP's comments are focused on Policy Relevant Background; new modeling, studies, and information which should be considered in assessing Policy Relevant Background; and the capabilities ofavailable models in relation to theformof a standard and Policy Relevant Background. We believe that EPA needs to reevaluate the PRBin the next draft ofthe ISA to address a number of issues to more properly characterize the PRB and the standard, notably that : 1) The level of the standard being contemplated is approaching non-controllable background levels. 2) Currently available modeling systems do not have the capability to simulate/capture the extreme events which is the formof the standard. 3) EPA's use of the models and the formof the standard needs to be bounded by the capabilities of the models and robust model performanceevaluation is needed. 4) EPA's current construct of Policy Relevant Background is not valid at thecurrent or contemplated level ofthe standard and needs to be extensively revisited with seasonality, geographical location, andstatistical formtaken into account. 5) Emissions fromCanada and Mexico, or any other country, should not be 

### considered controllable for US air quality regulations 

# MAY p5 2011

### 6) Anthropogenic emissions in the US necessary to maintain basic human and health services should be included in reevaluation of Policy Relevant Background instead of a zero emissions scenario. 7) There is a disconnect between the extreme formof the standard and the "average" formof the PRBthat must be reconciled. 8) There is no mechanismavailable for States to deal with the complex and overlapping causes for high ozone fromthe standpoint of "exceptional events" exclusion. This needs extensive work. 9) Hourly estimates of PRBshould be paired in space and time with observations for health risk assessments, and the effect of models underpredicting the frequency and magnitude of high ozone events on risk assessments must be quantified; As our detailed comments show, the current EPA-established PRBof 15-35 ppb is too low . It is our view

that EPA must allow

open scientific consideration of the issues presented by the PRB before any proposal for a new ozone NAAQS standard becomes final . BP provides a number of recommendations to informa PRB reevaluation. These comments were prepared by Nicole Downey, Ph.D. (Earth SystemSciences) ; Doug Blewitt, CCM

(Air Quality Resource Management) ; Dana Wood, PE (BP) . BP's detailed comments followthis cover letter. Thank you for your consideration of BP America's comments. Ifyou need any further information, you can contact me at sttohnk@bp.com(202-457-6594), or alternatively Gordon Smith at ordonreid .smith@bp.com(281-366-2683) . 

### Sincerely,



## . 

## Karen St.John Sr. Director, RegulatoryAffairs BPAmerica, Inc. 202-457-6594 (W) 202-351-1399 (cell)

## Comments on the `Integrated Science Assessment of Ozone and Related Photochemical Oxidants' EPA Document EPA/600/R-10/076A Released March 2011 Docket # EPA-HQ-ORD 2011-0187

May 5, 2011 Prepared by: Nicole Downey, Ph.D. Doug Blewitt, CCM Dana Wood, PE For BP America 

# bp

## 1 .0 Introduction EPA is charged with protecting human health, and the primary ozone standard is meant to reduce the risk ofnegative health effects due to tropospheric ozone pollution. Akey component of the standard-setting process, is calculating the benefit oftropospheric ozone reductions on human health relative to background ozone concentrations. EPA has defined Policy Relevant Background (PRB) concentrations as `those concentrations that would occur in the United States in the absence ofanthropogenic emissions in continental North America (defined here as the United States, Canada, and Mexico) . Policy Relevant Background concentrations include contributions fromnatural sources everywhere in the world and fromanthropogenic sources outside these three countries' (EPA 2006, 2011) . In their 2006 criteria document, EPAdefines PRB as 15-35 ppb (EPA 2006) based on modeling work by Fiore et al . (2002, 2003) . In the current ISA document, PRB is defined as 15-35 PPB based on the work of Fiore etal . (2002, 2003) along with an updated model run by Wang et al . (2009) . Here we argue that 1) The model runs reported by EPAare not the most recent simulations

of PRB . Newer model runs using the GEOS-Chem model have been completed for 2006-2008 and the results have been provided to EPA . These model runs were run at a higher spatial resolution, and model accuracy has been evaluated against CASTNET ozone observations across the country. Model simulations have shown an increase in predicted ozone across the entire US, but particularly in the Western US, compared to previous simulations of PRB . Additionally, PRB has been calculated using the CMAQand CAMx nested grid models at 12 kmresolution for 2006with GEOS-Chem boundary conditions. The newer PRB modeling evaluations indicate that model grid size is critical to simulate the highest ozone events in non-urban areas. This is especially true in elevated terrain in the intermountain West where smaller grid sizes are necessary to capture the processes responsible for PRB impacts in elevated terrain. Preliminary results fromthe CAMx simulation show that the 4th highest PRBvalue exceeds 60 ppb for -1/3 of the Western United States in 2006. The simulation shows that nearly '/Z ofthe United States (by area) has a 4`h highest PRB value greater than 50 ppb. 

### 2) While modeling is an important tool in estimating PRB, recent analyses ofair quality monitoring data have been conducted that are relevant to issues related to PRB . Thefollowing analyses should be cited and discussed in the ISA a. Jaffe (2010) - Relationship between Surface andFree Tropospheric Ozone in the Western US . Environmental Science and Technology.b. Lefohn et al . (submitted) - The importance ofstratospherictropospheric transport in affectingsurface ozone concentrations in the western and northern tier ofthe United States. Atmospheric Environment. c. Cooper et al . (2010) - Increasingspringtime ozone mixing ratios in thefree troposphere over western NorthAmerica. Nature. 

### These papers show

that there is significant correlation between ozone in the free troposphere/lower stratosphere and the surface, and that ozone in the free troposphere is increasing due to increases in Asian emissions. This national effect is most pronounced in theWestern United States (including California) where downwellingairmasses fromthe free troposphere transport ozone and ozone precursors to the surface for a large portion oftheyear. The recent Ca1Nex field campaign also foundsignificant impacts ofstratospheric intrusion and long-range transport ofemissions on both free tropospheric and surface ozone in California. Any literature recently published describing these results should be included in the ISA . 3) The results of the model runs EPA includes in the ISAare not represented accurately when comparedto the peer-reviewed publications they are derived from . EPA applies arguments related to the contribution of PRB ozone during events exceeding 85 ppb to all ozone levels. Fiore et al . (2002, 2003) and Wang et al . (2009) both argue that the contribution of PRBis highest between 50 and 70ppb, particularly in the intermountain West . EPA assumes that PRB decreases with high ozone, but this is not valid for ozone less than 75 ppb (fromFiore et al .) in the Eastern United States, and is not valid at any concentration range over the remainder ofthe US. In the most recent GEOS-Chemruns, decreasing ozone during high ozone events, is not observed at any ozone level (Zhang andJacob, in prep) . 4) EPA has failed to expand its conceptual models regarding causes of `high' ozone as it has adjusted the proposed standard downward over time from120 ppb (1-hour) to 85 ppb (8-hour) to 75 ppb (8-hour) to a stated level of 70to 60 ppb (8-hour) . The paradigms that explain `high' ozone formation at 85 ppb (8-hour) in low elevation urban areas are still valid; however if the standard is lowered, rural regions will exceed the standard. In both urban and rural areas, very different mechanisms explain `high' ozone at concentrations below 75 ppb. EPA must thoroughly re-evaluate its conceptual models regarding the causes of high ozone, when high ozone is defined below75 ppb. 5) Modeled PRBvalues used in health risk assessments must be paired in time andspace. Restricting PRBto mean values tends to underestimate the influence of PRB on high-ozone days, and therefore overestimate the health risk benefit of lowered ambient ozone. Using mean values for PRB masks potential model performance issues, creating more uncertainty with the model results . PRBvaries

### seasonally, by elevation and on an event basis (Fiore et al . 2002), and thesevariations must be taken into account in health risk assessments. Risk assessments should use PRBestimates at an hourly time scale. Monthly average hourly values are not sufficient. 6) EPA has defined PRBas a model derived value, but it has applied these values in ways that are not scientifically valid or technologically feasible. For discussions regarding attainment, PRB must be defined in a statistical form that is consistent with the standard. Restricting PRB to its current definition precludes using observational data to define PRB, but the models used to define PRB cannot capture high-ozone events in a manner consistent with the definition of the standard. Both the statistical formof PRB and the statistical form of the standard must be defined in a manner that is consistent with model performance limitations. Realistic analysis of the effects of model bias at high ozone must be conducted and included in the decision-making process. Models should be rigorously tested against observational data, and model performance evaluation should be focused on reproducing high-ozone events. Additionally, it should be recognized that the influence of background ozone on ambient ozone concentrations has significant interannual variability that may not be captured by a models run for a limited time period. Observational data, such as Jaffe et al . 2010 should be evaluated to estimate the magnitude ofthis uncertainty. 7) EPAjustified setting a relatively lowPRB in the 2006 Criteria Document based on the idea that PRBdecreases during high ozone events. EPA continues to argue that PRB decreases during high ozone events in the current ISA. This appears to be true for ozone events above -85 ppb in Eastern US urban areas. This is not, however, generally valid for ozone below75 ppb in the East, or ever valid over the remainder of the United States. Wang et al . (2009) clearly demonstrates that for ozone levels below75 ppb, there is no significant decrease with high ozone events. More recent work by Zhang and Jacob (in prep) shows that PRBincreases with higher ozone values in the Western United States during spring and summer, and in the Great Lakes region during spring. At no concentration range does the most recent GEOS-Chem modeling work showa significant decrease in PRB with and increase in ozone. 8) Defining PRB as the concentration ofozone that would exist in the absence ofanthropogenic emissions in the United States, Canada and Mexico implies that the United States can influence control of both Canadian and Mexican emissions. To date, no treaties exist between the United States and Canada or Mexico that make this possible. 9) EPA's focus on ozone levels in the absence ofanthropogenic emissions is outside the realmofpossible control, and should not be considered possible targets for compliance. Instead, realistic emission targets should be evaluated for optimized health benefits

### and emission reductions. These targets should be achievable in the 3-20 year time frame, and should include accounting for emissions required for maintaining basic public health services and a reasonable standard of living. Azero emission scenario would have negative public health impacts, which should be accounted for in the definition of PRBand the health benefits analysis. 10) Policy relevant background varies on a dynamic event level basis due to multiple factors including: upper tropospheric/lower stratospheric downwelling (UT/LS), biomass burning, lightning and long range transport of pollution plumes. The current regulatory framework available for compliance does not allow for exclusion of high background events. No regulatory mechanismexists for excluding high ozone events caused by downwelling of upper tropospheric air, ozone production by lightning, or a combination of multiple background influences that cause exceedance ofthe standard. 11) EPA fails to recognize that PRB varies significantly across the country, and that it is highly likely that significant portions of the United States cannot comply with a standard below75 ppb due to background ozone. A systemto identify and flag PRB events is essential for any standard below75 ppb, and no systemcurrently exists. For this reason, it is currently impossible to deal with PRB or exclusion of non-controllable high events in implementation. Observational data that could be used to identify background ozone is not available at most sites, and models cannot reliably simulate the highest ozone concentrations (and highest PRBimpacts) . The NAAQS ozone standard should be set within the valid range of models so that reliable and valid estimates of PRBevents can be generated. 

## 2.0 PRB simulations contained in ISAare outdated

EPA asserts that `the ISA accurately reflects "the latest scientific knowledge useful in indicating the kind and extent of identifiable effects on public health which may be expected fromthe presence of [a] pollutant in ambient air"' (ISA p 1-1) . In the case of policy relevant background (PRB) ozone, however, EPA has failed to consider the advancement ofglobal chemical 

### transport models, and has not included newer estimates of PRBderived from those advancements. Dr. Lin Zhang and Dr. Daniel Jacob (Harvard) have continued refining and testing the ozonesimulation in GEOS-Chem, and have recently completed a 3-year PRB run spanning 2006to 2008 using a nested-grid simulation at 0.5 x 0

.67 degrees over the contiguous United States.: 0.4 03 : 2^ d 0.2 : Q a 0.1 : 0.0 Spring Summer

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MDABozone jppbv] MDABozone [ppbvj Figure 1 . Distribution of MDA8 ozone concentrations for the standard (red) and improved (blue) GEOS-Chemozone simulations for 10 elevated CASTNET sites (>1 .5 km) in the Western US. Observational data are plotted in black. Notice that in Spring both models fail to capture the highest end of the probability distribution. Figure fromDr. Lin Zhang and Dr. Daniel Jacob (Harvard University) 

### Figure 1 shows the probability ofa given MDA8 ozonevalue occurring in observations (black), the standard GEOS-Chem model (red) and the latest GEOSChem model (blue) . Amodel bias of - 3 ppb in spring has been shifted to match observations . Notice, however, that the model still fails to capture the frequency and magnitude ofenhanced ozoneevents in spring (> 70 ppb), and it remains unclearwhether the mechanisms responsible for both mean and enhanced ozone events are accurately simulated in the model . This updated modeling data has been provided to EPA (Zhang and Jacob, personal communication) and represents the newer PRB estimation with the GEOS-Chem

model than what is contained in the current ISA. Important changes relative to the simulations of Wang et al . (2009) and Fiore et al . (2002, 2003) are: a new parameterization ofstratosphere to troposphere transport (STT), Asian emissions ofNO,, that are more consistent with satellite based observations, higher spatial resolution, updated emissions inventories, and full-year model runs. The GEOS-Chem

model continues to under-predict the frequency and magnitude ofenhanced ozone events at CASTNET sites above 70ppb (Figure 2), meaning that the model is biased lowat enhanced ozone levels. For example, at several sites in the Western US, measurements regularly exceed 60 ppb, but the model generally fails to capture these events (Figure 3) . This is likely due to underestimation ofstratospheric tropospheric transport in the model . Further work needs to be done to improve parameterizations ofthe variability in STT and it's contribution to enhanced ozone events.Number of days exceeding Wppbv 100 80 ta~GO C "N 40 c`~ 20 o ,

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10 20 90 <0 50 BO 0 5 10 15 20 CASTNet CASTNet Figure 2. Model performance evaluation ofthe GEOS-Chemmodel . This figure shows the frequency ofpredicted vs. observed MDA8 greater than 60, 65 and 70ppb for 12 CASTNET sites in the western United States. Notice that the model loses all predictive power at 70ppb in spring. Figure fromDr. Lin Zhang and Dr. Daniel Jacob (Harvard University) . Figure 2. Model performance evaluation of the GEOS-Chemmodel . This figure shows the frequency of predicted vs. observed MDA8 greater than 60, 65 and 70ppb for 12 CASTNET sites in the western United States. Notice that the model loses all predictive power at 70ppb in Spring. Figure fromDr . Lin Zhang and Dr. Daniel Jacob (Harvard University) . 100 BO 00 40 20 0 100 80 90 40 20 0 Yellows0orle, WY (45N,1101N, 2400m) Centennial, WY (41N, 108W, 3178m)

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Figure 3. Time series of observations, GEOS-Chem model predictions and predicted PRB at 4CASTNET sites in the western United States. Dashed line is at 60 ppb, notice that the model fails tocapture the highest events at all sites. Figure fromDr. Lin Zhang and Dr. DanielJacob (Harvard University) .p. 10



### The most recent GEOS-Chem simulations showthat modeled PRBcan be as high as 60ppb on an event basis (Figure 4), and given that the model tends to under predict the frequency and magnitude of high ozone events, it is reasonable to assume that actual PRBexceeds 60 ppb on an event basis. Importantly, the more recent GEOS-Chem simulation was conducted for an entire year. The impact of PRBpeaks in the spring, yet only summer was considered in Wang et al . (2009) . 

### Contribution fromthe US background ozone concentration US background ozone concentration estimated by a sensitivity study without US anthropogenic emissions US background ozonefor MDAa ozoneexceeding 60 ppbv SprinA Summer

0 12 24 36 4e so ippbvl Comparing with the US anthropogenic contribution (previous slide), ozone exceedance in the western US are more likely due to increases in the US background ozone. ,7 Figure 4 . a) mean PRBozone in Spring, b) mean PRB ozone in Summer, c) mean PRB ozone for days with MDA8greater than 60ppb in Spring, d) mean PRB ozone for days with MDA8greater than 60 ppb in Spring. Slide fromDr. Lin Zhang and Dr. Daniel Jacob (Harvard) 

### Additionally, Environ Corporation has completed a simulation of PRB using the CAMx and CMAQmodels at 12 km resolution, and the modeling protocol has been shared with EPA . It is important to note that the methodology used in that analysis is consistent with typical CAMx or CMAQapplications. Boundary conditions were established using GEOS-Chem fortheyear 2006and model predictions were then passed to 36and 12 kmCAMx and CMAQruns. Environ's results are consistent with those of Zhang andJacob (in prep), but they showa larger impact of biomass burning on PRB ozone (Figure 5) . These differences are driven by the smaller spatial resolution of the model, and byp. 11

the more-realistic temporal distribution of fire emissions. GEOS-Chem distributes a month's worth offire emissions evenly over the month, whereas CAMx uses a daily biomass-burning inventory to drive emissions. The CAMx implementation is more realistic, and points not only to the important role fires play in PRB ozone, but also highlights the importance of using higher resolution (in time and space) emission inventories. Preliminary results indicate that CAMx exhibits better model skill than GEOS-Chemand one of the reasons could be due to the higher resolution. Additionally, the maximumPRB observed in various models increases with decreasing grid size (60 ppb at 0.5 x 0.67 degrees, 84 ppb at 36 km, and 116 ppb at 12 km) . This illustrates that higher resolution models produce higher estimates of PRB, likely due to better representation of small-scale processes and higher resolution emission inventories.

Simulated annual 4t" highest NA background MOA8ozone GEOS-Chem

0 .5 x 0 .67 degree PRB (Harvard) CAMx 36 kmPRB(Environ) 90 .0 70 .0 80 .0 50 .0 40 .0 30 .0 20 .0 10.0 OA DDb

Figure S. Comparison of 4th highest annual average PRB estimates fromGEOS-Chem(Harvard) and CAMx (Environ) for the year 2006 . Notice that the maximum ofthe scale for the GEOS-Chemfigure is 60ppb, whereas the maximumfor the CAMx figures is 80 ppb. CAMx sees a much larger impact frombiomass burning in the west, likely due to differences in the temporal and spatial resolution of emission inventories. 

## 3.0 PRB values reported in ISAare not consistent with an ozone NAAQS below85 ppb. EPA asserts that Fiore et al . (2002, 2003) and Wang et al . (2009) support a PRB of 15-35 ppb. These arguments were formed around a NAAQS standardof 85 ppb, and were largely based on the observations in urban areas in the eastern United States. At very high ozone values (i .e. > 85 ppb), PRB ozone decreased and local emissions dominated ozone formation. In the classical ozone model, meteorological stagnation events lead to an inversion cap. Emissions ofozone precursors are trapped in a shallowboundary layer. Chemical reactivity rapidly increases and background ozone is rapidly depleted. High ozone develops after several days, and the high ozone event persists until a weather systemmoves through and breaks up the stagnation event (Logan et al . 1985) . This model works very well for describing the source of high ozone events > 85 ppb in the Eastern cities of the United States. It is not, however, an appropriate model to explain ozone in the 60-70 ppb range across the United States, especially in rural regions. Both Fiore et al . (2002, 2003) and Wang et al . (2009) found that PRB influence peaks between 50and 70 ppb ozone. These values are within the range being considered for the 2011 ozone reconsideration. A multitude offactors contribute to high background ozone in the 50 to 70 ppb range (See Section 9of this document), and most of these are not amenable to local or regional control strategies. 

### Fiore et al . (2003) specifically argues that PRB `should be represented as a function ofseason, altitude and total ozoneconcentration', and they found that in their coarse 2x2.5 degree model, PRBwas as high as 50ppb in the intermountain west in spring. Fiore et al . (2002) recognize that background ozone `represent[s] a major concern if the NAAQS were to be tightened' in the range of 50-70ppb. Wang et al . (2009) show event level PRB as high as 50 ppb at Whiteface Base, NY and Tucson, AZ (Figure 4 of that paper) . Wang et al . (2009) did not, however, calculate estimates for high-elevation sites or for spring, which leads to an underestimate of PRB in the intermountain west. Importantly, neither Fiore et al. (2002, 2003) or Wang et al . (2009) found that PRB decreases during high ozone events, when `high ozone' is defined in the 50-70 ppb range. This was a critical assumption of the 2006 Criteria Document, and remains an integral assumption of the 2011 ISA. EPA used this observation to justify setting a relatively lowPRBof 15-35 ppb, but this assumption is not valid in the 50-70ppb range. Additionally, recent work by Zhang andJacob (in prep) does not showa decrease in PRB at high ozone levels (Figure 6) . Instead, they see an increase in PRB with increasing ozone concentrations for high-elevation sites, which is consistent with the observations of Fiore et al . (2003) (Figure 7) .p. 13



### Spring Summer

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model calculated ozone vs. observations at CASTNET sites in 4 regions of the United States. Whisker plots represent the contribution ofNorthAmerican (blue) and US only background ozone for 10ppb bins. Notice that the contribution of background to high ozone does not decrease in any case. Also, notice that the contribution of background increases as ozone increases for the Intermountain West. Figure fromDr. Lin Zhang and Dr. Daniel Jacob (Harvard) . 

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### Cumulative probObility (90) Figure 7. Cumulative probability distributions for Elevated and Surface sites in the GEOS-Chemmodel developed by Fiore et aL (2003) . Black symbols are observations, red triangles are GEOS-Chempredictions, and green diamond are GEOS-Chemcalculated PRB. In this analysis, the contribution ofPRB increased with increasing ozone in Spring at Elevated sites, but decreased at the highest levels (generally greater than 80 ppb) . Notice thatthe model fails to reproduce the cumulative probability distribution ofthe measurements above - 60 ppb at elevated sites, and is biased low by -- 40 ppb at the highest end of the distribution for Summer at elevated sites. The model performs far better at surface sites. Figure taken from Fiore et al . 2003. 

## 4.0 For health effects studies, hourly estimates of PRB should be paired in space and time with observations EPA's current methodology for estimating the PRB contribution to the health effects of tropospheric ozone computes the difference between monthly mean diurnal cycles of PRBand observations. Because PRB varies significantly on an event basis, it is more appropriate to pair hourly PRBestimates with hourly observations in both space and time. EPA has been provided with hourly PRB estimates fromthe most recent 0.5 x 0.67 degree GEOS-ChemPRB model runs. For each city where health effects studies are conducted, an hourly time series of PRB at that location should be used. Additionally, because it is well documented that GEOS-Chemunder predicts the frequency of high 

### 12

### events, a detailed analysis of uncertainty must be conducted that quantifies the probable effect ofthis under prediction in health risk assessments. 

## 5.0 For discussions regarding probable attainment issues, PRB should be defined in a statistical formconsistent with the NAAQS standard Ofprimary importance in discussions regarding policy relevant background (PRB) is the statistical formchosen. NAAQScompliance is based on the 4th highest rolling 3-year average MDA8ozone value. This is roughly equivalent to the 99`h percentile ofthe ozone distribution. PRBvalues, however, have generally been discussed as a mean value. An illustration ofthe difference between the mean and extremevalues is shown in Figure 8 . Gothic, Colorado 15

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10 IWJ ILS 1006 lUUI IOUs 10L" J 101C 1011 -r-mear- -atn Mghest {-mean .one s~gma mean . 2,.grna Figure 8 . Mean, one sigma, twosigma and 4th highest MDA8values for the Gothic, Colorado CASTNET site. Mean MDA8 for 2005-2010was 50 ppb, mean + one sigma was 58 ppb, mean + two sigma was 67 ppb and the mean 4th highest MDA8 was 69 ppb. In this case, there is a 19 ppb difference between the mean and the 4th highest MDA8. 

### Here, we have plotted the mean, la, 2a and 4`h highest MDA8 for observations at the Gothic, Colorado CASTNET site for 2005 to 2010 . The Gothic, Colorado CASTNET site is located at an elevation of9,800feet in central Colorado and there are minimal local sources in the region. The average concentration is 50ppb, whereas the 4`h highest is approximately 69 ppb. The ISA presents modeling results for the mean and plus or minus one standard deviation fromthe mean. For this site, there is a 19 ppb difference between the mean and the fourth highest value. 

### 13

### Fiore et al . (2002, 2003), Wang et al . (2009) and the ISAdiscuss PRBin terms of means rather than extremes. It is, however, important to recognize that, with the current formofthe standard, the 4`h highest PRBvalue should be used to gauge the effects of background relative to the standard. Due to theglobal nature ofthe atmosphere, characterization of PRB must include both global chemical transport models (CTMs) . Because ofthe artificial definition of PRB, CTMs modeling must be used to quantify PRB, but only afterthe model accuracy has been compared against measurements at relatively remote sites to evaluate the accuracy their accuracy. CTM's, however, do not performperfectly at all sites, and significant discrepancies still exist between modeled and observed ambient ozone . Observational data (both in-situ andsatellite based) should be used to evaluate the large-scale processes such as STE, which appear to be significantly underestimated in models. CTM's, such as GEOS-Chem, tend to simulate mean conditions reasonably well, but fail at higher ozone values. As shown in Figure 7, the performance of GEOS-Chem degrades as ozone values increase, and the model estimates are up to 30ppb low for high ozone values at elevated sites in summer. Rastigejev et al . (2010) show that GEOS-Chem, and other Eulerian CTM's, are not capable of simulating the ozone distribution above the 95`h percentile (Jacob 2011), even if grid size is significantly decreased. Because models represent ensemble concentrations and because ofthe substantial uncertainties in model formulation and input data, models may never be able to accurately simulate extreme concentrations reliably. For example, numerical diffusion in Eulerian models (such as CMAQ, CAMx and GEOS-Chem) tends to disperse plumes far more rapidly than plumes actually disperse in the atmosphere, and a new generation of CTM's must be developed before these higher values can be properly accounted for in models. Fiore et al . (2002, 2003), Wang et ad. (2009) and Zhang andJacob (in prep) all state PRB never exceeds 60ppb in the model, but this may, in fact, be a model limitation. Model performance should be evaluated for both mean, and extreme observations, and a careful analysis detailing over what range each model is valid should be conducted. EPAshould, therefore, formulate PRB and the NAAQS standard in a manner that is consistent with the model performance limitations of the models that are used. Furthermore, models should be thoroughly evaluated against observational data (including ozone precursor species) at relatively remote sites, and uncertainty in model estimates should be quantified and taken into account in health effects calculations. 

## 6.0 Canadian and Mexican emissions should not be considered controllable by the United States. Policy Relevant Background is defined by EPA as the background ozone concentration in the U.S. resulting fromNorth American and transcontinental natural emissions as well as non-North American anthropogenic emissions. EPA 

### 14

### presumes that ozone precursor emissions fromCanada and Mexicowill or can be controllable to U

.S. emission standards (either by Canadian and Mexican regulations or by U

.S. sources paying for the installation ofemission controls on foreign sources) . Regulatory history has shownthat this presumption is not true. EPA states that "For this document, PRB (policy relevant background) concentrations include contributions fromnatural sources everywhere in the world and fromanthropogenic sources outside CNA (Continental North America) . Background concentrations so defined, facilitate separation of pollution that can be controlled by U.S. regulations or through international agreements with neighboring countries fromthose that arejudged to be generally uncontrollable by the U.S. Over time, consideration ofpotential broader ranging international agreements may lead to alternative determinations in which 03 precursor source contributions should be considered by EPA as part of PRB." To date there has been no international agreement entered in by the US that in any way requires or compels Mexico or Canada to control emissions or reduce emissions that are impacting the US. Under the La Paz Agreement of 1983 (http_//wwwepa gov/Border2012/docs/LaPazAareement .odt) Annex V establishes the Joint Advisory Council but it only has the power to "develop recommendations for the Air Work Group" (pg. 21) . The U.S.-Mexico Environmental Program, Border 2012, that resulted fromthe La Paz Agreement, has done nothing since its inception to improve air quality impacts fromMexico. 

### The preamble for the 2008Ozone NAAQS Reconsideration states "cross border 03 contributions fromwithin NorthAmerica (Canada and Mexico) entering the U .S. are generally thought to be small . Section 179B ofthe CleanAir Act allows designated nonattainment areas to petition EPAto consider whether such a locality might have met a clean air standard "but for" cross border contributions." However, the study "Surface ozone background in the United States: Canadian and Mexican pollution influences" by Wang et al . (2009) shows that "simulations for summer 2001 indicate mean North American and US background concentrations of26 _+ 8 ppb and 30 _+ 8 ppb, as obtained by eliminating anthropogenic emissions in North America vs. in the US only." "The Canadian and Mexican pollution enhancement averages 3 _+ 4 ppb in the US in summer but can be occasionally much higher in downwind regions of the northeast and southwest, peaking at 33 ppb in upstate New York (on a day with 75 ppb total ozone) and 18 ppb in southern California (on a day with 68 ppb total ozone) ." Furthermore, the study found that "exceedances of the 75 ppb US air quality standard in eastern Michigan, western New York, New

Jersey, and southern California are often associated with Canadian and Mexican pollution enhancements in excess of 10 ppb." In the 2009 analysis Wang concludes that "Unlike intercontinental transport that manifests itself mainly through enhancement of the hemispheric ozone background, pollution plumes from Canada and Mexico can be transported in the continental boundary layer to affect 

### 15p. 18

US areas immediately downwind" (Wang, 2009) . It should be noted that this study only considered impacts fromemissions originating in Mexico and Canada for the July through August (summer) time period and does not address potential influences during the March through May time period (spring) when many sites exhibit high ozone levels. This study needs to be extended and the influence of Mexican and Canadian emissions understood prior to a re-evaluation of PRB and setting of lower standards. It is also important for EPAto quantify the ozone effects fromemissions fromMexico and Canada to be considered consistent with the statistical formof the standard (3-year average of the 4`h highest daily 8-hour average) or on an event basis. Figure 9 (fromWang et al ., 2009) shows the JuneAugust mean daily 8-hour maximum ozone concentrations for North America background (PRB), US background, and the Canadian and Mexican enhancements. a North American background (PRB)

v

Canadian and Mexican pollution enhancement

Figure 9: June-Aug Mean Daily 8-hour Maximum Ozone Concentrations for a) PRB As defined in the ISA, b) ifemissions fromCanada and Mexico are included in PRB and c) the mean enhancement from Canadian and Mexican pollution (Figure taken from Wang, 2009) 

### 16

### As indicated in Figure 9, there is a mean enhancement ofdaily average 8-hr ozone concentrations during the summer of up to 15 ppb. However, compliance with the ozone standard is based on extreme concentrations (not mean concentrations) and to identify the actual ozone impact ofemissions fromMexico and Canada ofozone compliance in the US a more event driven analysis is required. EPA needs to completely analyze the impact of Mexican and Canadian emissions on US ozone levels in a comprehensive analysis that delineates: emission inventories used, meteorology used, model accuracy and model sensitivity (grid size etc.). Canadian and Mexican ozone contributions should be included in the quantification of PRB. In addition, EPA needs to identify what regulatory mechanismcan be used tojustify the assumption that Mexican and Canadian emissions will be controlled via. US emission standards. States are required to bring areas into NAAQS compliance via the SIP process, and states have no legal authority to forge international binding treaties, which would be necessary to control international emissions for US NAAQS. 

## 7.0 International emissions inventories and long-range transport driving PRB are highly uncertain Policy relevant background (PRB) ozone includes anthropogenic emissions of ozone precursors outside of North America. In general, the quality ofemissions inventories for countries outside ofNorth America and Europe is low, and a significant amount ofuncertainty exists regarding global NOX and VOC emissions. For example, Zhang et al . found that NOx emissions inventories for China, Japan and Korea were inconsistent with satellite-based observations, and that the emissions inventories required a factor of 2 increase to obtain agreement ofmodeling with observations. As the NAAQS ozone standard is decreased, understanding the contribution of international transport to ozone in the 50to 70 ppb range becomes extremely important. Cooper et al . (2010) found that ozone in the free troposphere has significantly increased over western North America, and they attribute this increase to rising Asian emissions. Currently, no-one has quantifiedthe uncertainty associated with global emission inventories on PRB estimates, particularly at an event scale. Observations at Mt. Batchelor, Oregon, suggest that long range transport ofAsian Pollution plumes impacts air coming into North America on a regular basis (Weiss-Penzias et al . 2006), and Obrist et al . (2008) saw

evidence of long range transport ofAsian pollution plumes at the StormPeak Lab in Colorado. Oltmans et al . (2010) found that biomass burning in Eurasia can enhance ozone by as much as 20 ppb in western North America from the high Arctic to the mid-latitudes. As discussed in Section 5, global chemical transport models (CTMs) disperse plumes more rapidly than actually occurs in the atmosphere due to numerical diffusion, so they are not sufficient tools to identify the sources of high ozone events. One reliable method for parsing out the sources 

### 17

### of plumes has been developed by Weiss-Penzias et al . (2007) . This method utilizes multiple species oftrace gases, which are simultaneously measured, and correlations between different species can be used to fingerprint sources of highozoneevents. For example, biomass burning plumes tend to be high in COand aerosols, whereas stratospheric intrusions are low in COand aerosols. These measurements are not available at AQMS across the United States, making it impossible to fingerprint sources of high ozone. Anetwork of monitoring sites that simultaneously measure multiple trace gases (such as the NCORE network) must be deployed, and must include at least onesite in the intermountain west (west ofthe continental divide) . At a minimum, the sites should measure 03, CO, aerosols andwater vapor. These sites will help to characterize the sources ofozoneas the standard approaches background levels, andwill be extremely valuable formodel performance evaluation. 

## 8.0 Stratospheric intrusions and downwelling ofupper tropospheric air masses remains a significant source of uncertainty in PRB simulations The current ISA asserts that "stratospheric intrusions that reach the surface are rare." Observational evidence, however suggests that stratospheric intrusions, folds, streamers and downwelling ofozone rich upper tropospheric air significantly contributes to surface ozone exceedance events. Models continue to underestimate the contribution ofthis effect to surface ozone concentrations. The recent Ca1Nex field campaign (2010) in California observed 8 stratospheric intrusions in 6 weeks (May 9 toJune 26, 2010) in California (Oltmans et al . 2010), and these events led to surface level MDA8 ozone > 75 ppb at at least one surface site (Langford et al . 2010) . Additionally, the downward mixing offree tropospheric air enriched with Asian emissions ofozone precursors ledto Paradise, CA exceeding an MDA8 of 75 ppb (Langfordet al . 2010) . Hocking et al . (2007, 608032) concluded that stratospheric intrusions can be relatively frequent and that intrusions can significantly impact ambient ozone concentrations. Langford et al . (2009) also observes that stratospherictropospheric-transport (STT) leads to exceedances of the NAAQS, which has also been observed by Lefohn et al . (2010, submitted), and it is clear that global CTM's do not reproduce the frequency or magnitude ofstratospheric intrusions and upper tropospheric downwelling events. Athorough analysis comparing observational evidence of STT, along with model predictions should be conducted. A technique to identify and flag UT/LS transport events based on widely available monitoring data must be developed and used for both implementation and model performance evaluation. 

## 9.0 Regulatory framework available for compliance does not take background into account. Under the current regulatory framework, there are two significant issues regarding PRB. The first is that high background events in excess ofthe standard cannot be excluded. There is the Exceptional Events requirements of 40CFR50.14, but 18p. 21

that restricts flagging data to ozone exceedances that are frombiomass burning, long range transport, or stratospheric intrusion. There is no mechanismto deal with downwelling of free tropospheric air or high ozone that is associated with lightning. Additionally, ifa high background event is caused by multiple sources, including long range transport and natural ozone production, there is no legal mechanismto remove the data for attainment status. 40 CFR 50.14(c)(3)(iv) requires that "There would have been no exceedance or violations but for the event" which means there can only be one source. Further more, the definition of an "Exception Event" (40CFR 50.1(j)) states that it "Is an event caused by human activity that is unlikely to recur at a particular location or a natural event," meaning that events that continue to occur fromtransport ofanthropogenic emissions could not be excluded. 

### Thesecond significant issue is that ozonecontrols available at a state or county level are insufficient to demonstrate compliance with a standard that is very near background. Even with no local sources, many locations will exceed attainment dueto transport of non-local, domestically produced ozone and ozone precursors. Multi-state and national coalitions ofstate governments and industry, such as the Four Corners Task Force, will be necessary to meet attainment goals. 

## 10.0 The technological tools to deal with PRB in implementation do not exist. If the NAAQS ozone standard is tightened closer to background, it will become necessary to develop tools to filter out PRBevents. The change in the frequency ofgrid cells with ozone exceedances predicted by the 0.5 x 0.67 degree GEOSChem model increases nearly exponentially after 75 ppb (Figure 10) .



### < i

~ .i U ; tL"1 Q 2 U L O_` i 4000 30'~'J

, i)'. 7

ts~ 75 7C- 65 '-"D ()/:~'IH ( :;Pt,~ =k< Figure 10

. Number ofgrid boxes in the Harvard GEOS-ChemPRB full emissions simulation that exceed a 4`h highest MDA8 of60ppb . This figure was generated frommodel runs conducted at Harvard, which were provided to EPA. 

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